The present invention relates to a data processing apparatus for a chromatograph and, more particularly, to a method of correcting a base line.
According to Japanese Patent No. 1,385,425 (Japanese Patent Publication No. 61-54181), as a method of correcting a base line of non-separation peaks, there are: (1) the N method, (2) the xcex8 method, and (3) the method of merely connecting a base portion by a straight line (non-correction method).
In the N method, the number of peaks (N) is designated, N peaks are collected as one peak, and either a base portion or a trough portion is connected by a straight line (FIG. 3(A)). The base portion is a portion that is judged as not a peak area by using a change amount of signal as an index.
In the xcex8 method, an inclination is intentionally loosened when the inclination seems to be too steep in the N method (FIG. 3(B)).
The non-correction method is a most typical method and is effective when the base line is estimated to be like a straight line (FIG. 3C). In addition, there is also a case of correcting a base line by intentionally selecting a forward horizon method (FIG. 3D), a backward horizon method (FIG. 3E), a special processing method of a shoulder peak (FIG. 3F), or the like in accordance with a peak shape of each chromatogram.
Each of the above base line correcting methods has both merits and demerits and is selected depending on a particular case, since it is difficult to unconditionally determine the base line. Although some methods in which the base line is unconditionally determined have been proposed (Japanese Patent Application Laid-Open Sho 62-32360, Japanese Patent Application Laid-Open Sho 63-88443, Japanese Patent Application Laid-Open Hei 6-94696, and the like), those methods are not yet generally used.
In all of the above methods, the base line is corrected on the basis of the base or trough portion existing in the chromatogram.
An algorithm of searching the base or trough portion generally tends to be too sensitive to a local fluctuation in a signal. For example, a case where the trough portion is detected and a case where it is not detected occur depending on a magnitude of noise. A detection of a starting point and an ending point of a peak, that is, an end point detection of the base portion is also disturbed by noise. After all, such a correction of the base line is easily affected by noise and quite different base lines may be obtained due to a slight difference in signals.
By a similar reason; there is a case where the base line largely fluctuates when parameters such as sensitivity, slope, and the like, to detect the base portion or trough portion, are improper.
In the case where the base line is experientially estimated to be horizontal, if a horizontal straight line is applied to the base portion, a proper base line can be obtained. In the case where the base line may possibly not be horizontal, however, a method of obtaining the base line by continuing the base or trough portion with a straight line like a graph of polygonal line is not always proper.
It is an object of the invention to provide a data processing apparatus for a chromatograph, which solves the above problems, forms a stable base line without being affected by a local noise of a chromatogram, and corrects the base line irrespective of local fluctuations of a chromatograph.
In a data processing apparatus for a chromatograph comprising base line determining means for detecting an output value of a detector for a chromatograph obtained with the elapse of time, forming a chromatogram on the basis of the detection result, and determining a base line on the basis of a shape of the chromatogram, when the base line is corrected, a deviation in the direction of the output value between each of forming points which form the base line and a forming point adjacent to the forming point is largely reduced as the deviation becomes larger.
Adjusting means of the base line adjusts so as to reduce a change amount of the deviation proportional to the deviation with an increase in the deviation and to increase the change amount of the deviation proportional to the deviation with a decrease in the deviation.
Further, an adjusting range of the adjusting means from a base line is determined on the basis of the shape of the chromatogram to a linear base line connecting base lines before and after a time zone in which the chromatogram appears as a measuring target appears.
There is also provided selecting means for applying the above construction to an optional time zone of the measurer.
In the time zone in which the chromatogram appears as a measuring target sample, the deciding means for deciding the base line on the basis of the shape of the chromatogram decides the base line on the basis of a distance on the same time base from either one of a straight line connecting a starting point and an ending point of the time zone in which the chromatogram appears, a straight line connecting the starting point and the trough portion of the chromatogram, and a straight line connecting the ending point and the trough portion of the chromatogram to the chromatogram, and arranges a forming point of the base line so as to shorten a distance on the same time base from the straight line for the distance to the base line with an increase in the foregoing distance.
In one mode of the arrangement of forming points on the base line, the base line is set to the same line as the chromatogram in the time zone in which the foregoing distance is equal to a value which ranges from zero to a predetermined value.
The base line of the chromatogram inherently has a very loose change as compared with the signal change in the peak area and its change width is narrow.
That is, when the base line is regarded as a collection of a plurality of points, it is considered that two neighboring points among the points are loosely bound to each other. Namely, a base line of a shape having a local projection is impossible. If the base line having a local projection is drawn, it is considered that the portion is projected due to improper drawing means of the base line or an influence by noise occurring somewhere in a chromatograph device.
In consideration of the above, when the base line drawn by various means is regarded as one line constructed by a plurality of points, the correction is performed so as to reduce a deviation in the direction of the output value between the two adjacent points among the points as the deviation becomes large, so that the base line adapted to the above condition can be drawn.
The base line drawn as mentioned above can be adjusted while maintaining a gentle line in a manner such that when the deviation between the two points in a time zone is increased, a change amount of the deviation is reduced in proportion to the increase in the deviation and, when the deviation is reduced, the change amount of the deviation is increased in proportion to the decrease in the deviation.
Another characteristic of the base line is such that the base line is strongly attracted to an area in which a signal value expressed by the chromatogram is small.
It is because the signal value that is caused by another component which is a factor of the noise is unlikely to appear in the time zone in which a target component appears and an output value which is not so large is generated since it corresponds to an amount of noise after all, and the like.
In consideration of the above, the base line corresponding to the above characteristic can be drawn by arranging the forming points of the base line in a manner such that as the distance in the time base direction from the chromatogram to the base line as a reference (linear base line connecting base lines before and after the time zone in which the chromatogram as a measuring target appears) becomes longer, a distance in the same time zone from the straight line for the foregoing distance to the base line is largely reduced.
When the drawing means of the base line and the correcting means of the base line are used together, since the projected portion of the base line drawn by the drawing means can be corrected by the correcting means of the base line, the correction of the base line in which the two characteristics of the base line are considered can be performed.